Children playing online games such as Candy Crush Saga and virtual poker could become gamblers when they grow older, according to an expert on the gambling industry.

Mark Griffiths, director of the International Gaming Research Unit at Nottingham Trent University, told the Times Education Supplement: "It's a bit like the old drug-dealing analogy of giving a bit for free and hooking them in. Games like Candy Crush have a moreishness quality, a bit like chocolate. You say you you'll just have one chunk, and you end up having the whole lot. So you say I'll just play for 15 minutes, and you end up still there four or five hours later."

Candy Crush Saga has been downloaded 500m times. Its owners say it is aimed women between the ages of 35 and 50. It is possible to play the game for free but 40% of players pay for add-ons.

Griffiths said there was an overlap between online games that allowed players to spend money on virtual accessories or to access higher levels, and gambling. "It's a psychological masterstroke that people pay money to buy virtual items. The next step is for gambling firms to say, maybe you could win back some of the money you're spending," he said.

According to Griffiths, online games with virtual money lead children to gambling. "Children who play these free games are more likely to gamble and more likely to develop problem gambling behaviours. These are gateway activities that can lead people down the gambling road. When you start winning, you start thinking that if I was playing with real money I could be doing quite well."

Griffiths said children needed to be educated about the pressure of advertising and online games. "I don't want to come across as an omen of doom. There's nothing wrong with kids playing gambling-type games, but you have to accompany it with education."